1. Field of the Invention
This invention relates to a hybrid vehicle of the type driven by a combination of an internal combustion engine (hereinafter referred to as "engine") and an electric motor (hereinafter referred to as "motor").
2. Description of the Related Art
Conventional hybrid vehicles can be generally classified into two groups, the first being a series-type hybrid vehicle in which an engine drives an electric generator and a motor is driven by the electricity to drive the wheels and the second being a parallel-type hybrid vehicle in which the wheels are directly driven by an engine and/or a motor. The hybrid vehicle of the series type is characterized in that the engine is installed separate from the drive train to allow the engine to be run at peak efficiency.
The parallel-type hybrid vehicle can attain an effective energy transmission with a minimum of energy conversion from mechanical to electrical, since the main portion of the drive torque is generated by an engine and an auxiliary torque is obtained from a motor. However, it has been known in the art that engine speed in the parallel-type hybrid vehicle naturally deviates from the most efficient speed because it corresponds to the vehicle speed.
Japanese Utility-model Application No. Hei 2-7702 proposes a modified hybrid vehicle wherein the engine and generator are optionally disconnected from an output shaft by means of a clutch. When the clutch is engaged the vehicle operates as a parallel-type hybrid vehicle, and when the clutch is released the vehicle operates as a series-type hybrid vehicle. It is therefore possible to release the clutch when the vehicle travels on city streets and to engage the clutch when the vehicle moves at high speed.
However, in such conventional hybrid vehicles, the engine and the motor are arranged on one single axis, so that the whole drive system tends to require a long space along that axis. In a FF-type hybrid vehicle, such a long drive train reduces space for the steering angle and thereby increases the minimum turning radius. Furthermore, because rotary power from both the engine and the motor is output to a common output shaft, the gear ratio for the engine should be the same as that for the motor. Therefore, optimum gear ratios for the engine and the motor cannot be independently determined in designing.